A precision approach path indicator, PAPI in short, is a light system, which is installed on the ground adjacent to a runway for providing visual indication of the position of an aircraft position on the glide path for approaching the runway in question. The PAPI is typically positioned on the left side of the runway so as to be seen up to eight kilometers in daylight and 32 kilometers in the dark. A conventional PAPI includes an optical arrangement configured to divide light output into two distinctive segments, red and white. Depending on the viewing angle during approach, the lights will appear either red or white. The color of the light, as seen by the pilot, conveys information concerning the conformity to the appropriate glide path for the aircraft. When there is an even split in red and white lights, the aircraft is on the correct glide path. If an aircraft is beneath the correct glide path, red lights will outnumber the white. Alternatively, if an aircraft is above the correct glide path, white lights outnumber the red.
PAPI's have conventionally employed artificial light sources that are based on incandescent light bulbs. As such bulbs suffer from poor efficiency and short life span, there has been a need to develop PAPI's running more efficient and robust light sources. PAPI's running light-emitting diodes (LED) have therefore been introduced. US 2007/0115141 A1, for example, discloses a PAPI with two LED arrays (16, 18) arranged at a distance from each other in an opposing manner to emit white and red light toward each other in one direction. Each individual LED includes a collimating or encapsulating optic between the opposing LED arrays (16, 18) is a mirror component (14) having a two planar mirror surfaces (32, 34) arranged in a respectively 90 degree angle so as to lie in a 45 degree angle relative to the LED arrays (16, 18). The planar mirror surfaces (32, 34) reflect the white and red light from the LED arrays (16, 18) toward a projection lens (22).
As the requirements for the light output of PAPI's is very strict, there remains a need to further develop the distribution of light intensity in a given glide angle. While it is necessary to satisfy the requirements of standards governing lines of constant light intensity, i.e. isocandela lines, it is desirable to pursue ever greater fidelity and sharpness in the distribution of light. One such standard is Annex 14 to the Convention International Civil Aviation (“Aerodromes Volume I—Aerodrome Design and Operations”, Fifth Edition, July 2009 by the International Civil Aviation Organization). Because the source of light in an array of LEDs is not a point source but rather each array has a plurality of light sources, the required optics tend to be quite large to capture and guide the light in a desired fashion.
It is therefore an object of certain embodiments to provide a PAPI having a semiconductor light source that is not only compact in size but also has sharp distribution of lights.